This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations involving the use of certain amphoteric surfactants which are suitable for use in brines containing relatively high concentrations of divalent metal ions.
In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into the reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus it has been proposed to add surfactants to the flood water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add viscosifiers such as polymeric thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
Processes which involve the injection of aqueous surfactant solutions are commonly referred to as surfactant waterflooding or as low tension waterflooding, the latter term having reference to the mechanism involving the reduction of the oil-water interfacial tension. Thus far, many such waterflooding applications have employed anionic surfactants. For example, a paper by W. R. Foster entitled "A Low-Tension Waterflooding Process", Journal of Petroleum Technology, Vol. 25, Feb. 1973, pp. 205-210, describes a promising technique involving the injection of an aqueous solution of petroleum sulfonates within designated equivalent weight ranges and under controlled conditions of salinity. The petroleum sulfonate slug is followed by a thickened water slug which contains a viscosifier such as a water-soluble biopolymer in a graded concentration in order to provide a maximum viscosity greater than the viscosity of the reservoir oil and a terminal viscosity near that of water. This thickened water slug is then followed by a driving fluid such as a field brine which is injected as necessary to carry the process to conclusion.
One problem encountered in waterflooding with certain of the anionic surfactants such as the petroleum sulfonates is the lack of stability of these surfactants in so-called "high brine" environments. These surfactants tend to precipitate from solution in the presence of monovalent salts such as sodium chloride in concentrations in excess of about 2-3 weight percent and in the presence of much lower concentrations of divalent metal ions such as calcium and magnesium ions. Typically, divalent metal ion concentrations of about 50-100 ppm and above tend to cause precipitation of the petroleum sulfonates. The salinity of the surfactant slug is also significant with regard to interfacial tensions achieved through the use of petroleum sulfonates such as disclosed in the Foster paper. Even in the absence of divalent metal ions, optimum interfacial tensions are seldom achieved at salinities significantly in excess of 2-3 weight percent.
Various surfactant formulations which will tolerate high salinities and/or high divalent metal concentrations have been proposed for use in high brine environments. Thus, U.S. Pat. No. 3,939,911 to Maddox et al. discloses a surfactant waterflooding process employing a three-component surfactant system which tolerates polyvalent ion concentrations from about 200 to about 14,000 parts per million. The three-component surfactant system includes an alkyl or alkylaryl sulfonate such as an ammonium dodecyl benzene sulfonate, a phosphate ester sulfonate, and a sulfonated betaine such as C.sub.12 -C.sub.24 alkyl amido C.sub.1 -C.sub.5 alkane dimethylammonium propane sulfonate. The several surfactant components may be employed in concentrations from about 0.05% to about 5.0% and preferably about 0.2% to about 0.5% by weight. The surfactant system is said to be stable up to at least 225.degree. F. and to be resistant to bacterial attack and inhibits scale formation.